Uncovering the creep deformation mechanism of rock-forming minerals using nanoindentation

The creep phenomenon of rocks is quite complex and the creep mechanisms are far from being well understood. Although laboratory creep tests have been carried out to determine the creep deformation of various rocks, these tests are expensive and time-consuming. Nanoindentation creep tests, as an alternative method, can be performed to investigate the mechanical and viscoelastic properties of granite samples. In this study, the reduced Young's modulus, hardness, fracture toughness, creep strain rate, stress exponent, activation volume and maximum creep displacement of common rock-forming minerals of granite were calculated from nanoindentation results. It was found that the hardness decreases with the increase of holding time and the initial decrease in hardness was swift, and then it decreased slowly. The stress exponent values obtained were in the range from 4.5 to 22.9, which indicates that dislocation climb is the creep deformation mechanism. In addition, fracture toughness of granite's rock-forming minerals was calculated using energy-based method and homogenization method was adopted to upscale the micro-scale mechanical properties to macro-scale mechanical properties. Last but not least, both three-element Voigt model and Burgers model fit the nanoindentation creep curves well. This study is beneficial to the understanding of the long-term mechanical properties of rock samples from a micro-scale perspective, which is of great significance to the understanding of localized deformation processes of rocks.